The present invention relates to an image forming apparatus and an image forming method for suppressing the occurrence of the degradation factors of the image quality such as a banding phenomenon occurring in the secondary scanning direction during image formation on an image carrier.
An image forming apparatus comprises a plurality of image formation units, such as an image carrier, an image writer, a charger, a developer, and a transferee. In such an image forming apparatus, the initiation and the termination of each of the operations for rotating the image carrier, charging the image carrier, exposing the image carrier (latent image writing), developing the latent image and transferring a toner image are prescribed as a sequence. Incidentally, vibrations may occur in the apparatus in synchronization with various steps in the sequence. Further, the circumferential speed of the image carrier may fluctuate.
In the case of the image forming apparatus is a monochromatic printer, as shown in FIG. 20, a rotation signal for an image carrier turns ON at time “to”. At the same time, a charging signal turns ON. Then, at time “ta”, a signal for image writer turns ON, so that exposure operation begins.
After that, at time “tb”, an activation signal for a developer turns ON. The activation of the developer indicates such time duration that the developer is performing any operation such as the rotation of a development roller. Then, at time “tc”, an activation signal for a transfer roller turns ON, so that transfer operation begins. The transfer operation includes the operation of the transfer roller and the application of a transfer bias. At time “td”, the rotation signal for the image carrier turns OFF, so that the transfer operation ends.
In this example, vibrations in the apparatus and a fluctuation in the circumferential speed of the image carrier occur at the timing that the operation of the developer is initiated or terminated. The fluctuation in the circumferential speed of the image carrier is caused by an increase or a decrease in the friction in the image carrier. These increases and decreases are caused by a fluctuation occurring at the above-mentioned timing in an electrostatic force associated with the bias (voltage). In a case where this timing of initiation or termination of the development falls within the duration of the exposure operation as indicated by time “tb”, inhomogeneity occurs in the image density at that timing. Further, in case where the transfer signal turns ON during the exposure operation as indicated by time “tc”, vibrations occur also at this timing, so that inhomogeneity occurs in the image density.
FIG. 21 shows a case where the image forming apparatus is a four-cycle color printer which forms a full color image by overlaying toners of four colors of yellow (Y), cyan (C), magenta (M), and black (K). Also in this case, a rotation signal for an image carrier turns ON at time “to”. At the same time, a charging signal turns ON. Then, at time “ta”, exposure operation begins. At time “tr”, development operation begins. At time “ts”, transfer operation begins. In such a four-cycle color printer, these exposure operation, development operation, and transfer operation are repeated on the same image carrier four times.
Also in this example, the development operation is initiated or terminated during the exposure operation at time “tr” and “tu”, respectively. Accordingly, vibrations in the apparatus and a fluctuation in the circumferential speed of the image carrier occur at these timings. Further, since the transfer signal turns ON and OFF respectively at time “ts” and “tv” during the exposure operation, vibrations occur in the apparatus also at these timings. This causes color shift (misalignment) in the color image formation, and hence degrades the quality.
FIG. 22 shows a case where the image forming apparatus is a tandem type color printer in which image carriers of black (K), cyan (C), magenta (M), and yellow (Y) are arranged so as to form an image by overlaying these colors successively. At time “th” exposure operation is initiated. At time “ti” development operation is initiated. At time “tj”, primary transfer operation turns ON (c). In such a tandem type color printer, these kinds of operations are performed in parallel for the four colors.
At time “tk”, secondary transfer operation is initiated, while at time “tm”, the primary transfer operation is terminated. Further, at time “tn”, the secondary transfer operation is terminated. Also in the secondary transfer operation, vibrations or the like occur owing to driving operation for the transfer roller and paper feed operation. Thus, similarly to the cases of the development operation and the primary transfer operation, in a case where an ON or OFF signal for the secondary transfer is generated during the exposure operation, color shift occurs and degrades the printing quality.
The above-mentioned vibrations in the apparatus and the fluctuation in the circumferential speed of the image carrier occurring in synchronization with various steps in the sequence of the operation of the image forming apparatus have degraded the precision in the exposure position and hence have caused the problem of density inhomogeneity (banding phenomenon). Further, in the color printers in which a plurality of colors are overlaid, color shift or the like has been caused by the vibrations and the fluctuation described above. In short, the prior art has the problem that the vibrations in the apparatus and the fluctuation in the circumferential speed of the image carrier occurring in synchronization with various steps in the sequence of the operation of the image forming apparatus degrade the printing quality.
In such an image forming apparatus, when the image carrier is driven by a drive motor, the circumferential speed fluctuation is caused by the gear tooth pitch of a gear wheel linked with the image carrier.
FIG. 23 shows the characteristics of the circumferential speed fluctuation in the image carrier and the size of the image generated by exposure. The horizontal axis indicates time T, while the vertical axis indicates the circumferential speed V of the image carrier. Va indicates a normal value for the circumferential speed. The circumferential speed varies in the form of a sine wave having a period of t1 owing to the rotation error caused by the gear tooth pitch of the gear wheel linked with the image carrier.
As time advances, the circumferential speed increases starting at the normal value Va. The circumferential speed reaches the maximum Vb at time “ta1”. After that, the circumferential speed decreases as time advances. The circumferential speed returns to the normal value Va at time “tb1”. Then, as time advances, the circumferential speed further decreases from the normal value Va. The circumferential speed reaches the minimum Vc at time “tc1”. After that, the circumferential speed returns to the normal value Va at time “td1”. A half of the cycle of the circumferential speed fluctuation is designated as t2.
When the circumferential speed is at the normal value Va, an image Gx is formed in a normal circular shape. When the circumferential speed is at the maximum Vb, an image Ga is formed in an elliptical shape having the major axis in the secondary scanning direction, and has a larger exposure area than the normal case. When the circumferential speed is at the minimum Vc, an image Gb is formed in an elliptical shape having the major axis in the primary scanning direction, and has a smaller exposure area than the normal case.
As such, the inhomogeneity in the circumferential speed of the image carrier caused by the gear tooth pitch results in density inhomogeneity. This is because: (1) the inhomogeneity in the circumferential speed of the image carrier causes a difference between the spot shapes at an exposure position having a higher circumferential speed and at an exposure position having a lower circumferential speed; and (2) these spots have a difference in the exposure energy per unit area. These two reasons (1) and (2) cause the density inhomogeneity. Further, in the case of a color printer for overlaying a plurality of colors, color shift is caused.
FIG. 24A shows an example of image formation in a case where a fluctuation occurs in the circumferential speed of the image carrier as described with reference to FIG. 23. Symbol X denotes the paper feeding direction (secondary scanning direction).
A normal image Gx is formed in the pixel line Sa at a certain time. After the image carrier travels in the direction indicated by X, the next pixel line Sb reaches the exposure position. At that time, the circumferential speed has increased from Va to Vb. Thus, an image Ga is formed in an elliptical shape having the major axis in the secondary scanning direction. After the image carrier further travels in the direction X, the next pixel line Sc reaches the exposure position. At that time, the circumferential speed has decreased from the Vb to the normal value Va. Thus, a normal image Gx is formed in the pixel line Sc.
After the image carrier further travels in the direction X, the next pixel line Sd reaches the exposure position. At that time, the circumferential speed has decreased from Va to Vc. Thus, an image Gb is formed in an elliptical shape having the major axis in the primary scanning direction. In subsequent processes, a series of images Gx-Ga-Gx-Gb-Gx is periodically repeated. This indicates the occurrence of density inhomogeneity. FIG. 24B shows an example where a normal image Gx is formed in each of pixel lines Sa-Si for the comparison purpose.
In view of the above, Japanese Patent Publication No. 2000-98802A discloses that a flywheel is provided coaxially to a rotary shaft of an image carrier, and that the flywheel is arranged so as to rotate in association with the rotation of the image carrier. By virtue of this, the inertial moment generated by the rotating flywheel suppresses the velocity fluctuation, so that the density inhomogeneity is reduced.
Japanese Patent Publication No. 2000-112196A teaches that a viscous-fluid damper is provided in a rotary shaft so as to avoid the influence of the vibrations of the apparatus.
Japanese Patent Publication No. 2000-89640A teaches that the rotation of the image carrier is monitored by an encoder, and that the monitored value is compared with a reference value, so that the light emission timing is controlled.
The configuration disclosed in Japanese Patent Publication No. 2000-98802A causes an increase in the number of components. Further, the flywheel having significant size and weight causes an increase in the size and the weight of the apparatus. Similarly, the configuration disclosed in Japanese Patent Publication No. 2000-112196A causes an increase in the number of components, and hence unavoidably results in a cost increase. In both of these documents, a countermeasure is provided in the image carrier. Although the vibrations of the apparatus transmit through also to an image writing head, no proposal has so far been made where a countermeasure against the vibrations is provided in the image writing head. This has avoided a satisfactory solution to the problem of printing quality degradation.
The configuration disclosed in Japanese Patent Publication No. 2000-89640A causes unavoidably an increase in the number of components. Further, precise attachment of the encoder is difficult, and that complicated control is necessary in the timing control.